This invention relates to the field of photographic reproduction, and specifically comprises an inexpensive accessory for extending the usefulness of a known photolithographic plate making machine inexpensively so that it can do the work of a second expensive machine.
The principal work of a photolithography establishment is producing multiple copies of original sheets containing "line" material, that is, typescripts, charts, diagrams, and line drawings. For this purpose, a light sensitive photolithographic medium is used comprising firm paper coated with a complex emulsion such that after exposure and developing the portions of the medium, hereafter called a "plate," which are black in the original, have an affinity for ink, while the remainder of the plate does not. The plate may now be used for offset printing in the usual fashion, to make as many copies as are desired.
Equipment is known for making exposure on such medium and automatically developing the resulting plates. The medium, the appropriate activating and fixing chemicals, and the processing times and temperatures are all familiar. Reference is made to Fairbanks U.S. Pat. No. 3,283,647 for a teaching in this area.
There is, however, a significant demand in such establishments for copies of material which is not susceptible of treatment as described above. Photographs, three dimensional objects, and generally any material where shades of gray must be reproduced, do not give plates, using the medium and processing described above, which can be used satisfactorily for offset printing. A different light sensitive medium must be used, and the copying must be done through a half-tone screen to give an intermediate copy known as a "screen print," which itself can subsequently be copied by the basic process first described to give a satisfactory plate. The medium and processing chemicals and procedures are again known, as are machines for doing this work automatically, but the cost of such machines is so great that unless the purchaser can use the machine a high percentage of his daily working hours, it is not economically feasible.